Commit 96312e61 authored by Andrea Arcangeli's avatar Andrea Arcangeli Committed by Linus Torvalds
Browse files

mm/gup.c: teach get_user_pages_unlocked to handle FOLL_NOWAIT

KVM is hanging during postcopy live migration with userfaultfd because
get_user_pages_unlocked is not capable to handle FOLL_NOWAIT.

Earlier FOLL_NOWAIT was only ever passed to get_user_pages.

Specifically faultin_page (the callee of get_user_pages_unlocked caller)
doesn't know that if FAULT_FLAG_RETRY_NOWAIT was set in the page fault
flags, when VM_FAULT_RETRY is returned, the mmap_sem wasn't actually
released (even if nonblocking is not NULL).  So it sets *nonblocking to
zero and the caller won't release the mmap_sem thinking it was already
released, but it wasn't because of FOLL_NOWAIT.

Fixes: ce53053c

 ("kvm: switch get_user_page_nowait() to get_user_pages_unlocked()")
Signed-off-by: default avatarAndrea Arcangeli <>
Reported-by: default avatarDr. David Alan Gilbert <>
Tested-by: default avatarDr. David Alan Gilbert <>
Cc: Al Viro <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 1b4cfe3c
......@@ -516,7 +516,7 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
if (ret & VM_FAULT_RETRY) {
if (nonblocking)
if (nonblocking && !(fault_flags & FAULT_FLAG_RETRY_NOWAIT))
*nonblocking = 0;
return -EBUSY;
......@@ -890,7 +890,10 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
if (*locked) {
/* VM_FAULT_RETRY didn't trigger */
* VM_FAULT_RETRY didn't trigger or it was a
if (!pages_done)
pages_done = ret;
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